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Abstract

Spontaneous teratocarcinomas are ovarian or testicular tumors which have
their origins in germ cells. The tumors contain a disorganized array of benign
differentiated cells as well as an undifferentiated population of malignant
stem cells, the embryonal carcinoma or EC cells. These pluripotent stem cells
in tissue culture share many properties with the transient pluripotent cells of
the early embryo, and might therefore serve as models for the investigation of
developmental events ill vitro.
The property of EC cells of prime interest in this study is an in vivo
phenomenon. Certain EC cell lines are known to be regulated ill vivo and to
differentiate normally in association with normal embryonic cells, resulting in
chimeric mice. These mice have two genetically distinct cell populations, one
of which is derived from the originally malignant EC cells. This has usually
been accomplished by injection of the EC cells into the Day 3 blastocyst. In
this study, the interactions between earlier stage embryos and EC cells have
been tested by aggregating clumps of EC cells with Day 2 embryos. The few
previous aggregation studies produced a high degree of abnormality in chimeric
embryos, but the EC cells employed had known chromosomal abnormalities. In this
study, two diploid EC cell lines (P19 and Pi0) were aggregated with 2.5 day
mouse embryos, and were found to behave quite differently in the embryonic
environment. P19 containing aggregates generally resorbed early, and the few
embryos recovered at midgestation were normal and non-chimeric. Pi0 containing
aggregates survived in high numbers to midgestation, and the Pi0 cells were very
successful in colonizing the embryo. All these embryos were chimeric, and the contribution by the EC cells to each chimera was very high. However, these
heavily chimeric embryos were all abnormal.
Blastocyst injection had previously produced some abnormal embryos with
high Pl0 contributions in addition to the live born mice, which had lower EC
contributions. This study now adds more support to the hypothesis that high EC
contributions may be incompatible with normal development. The possibility that
the abnormalities were due to the mixing of temporally asynchronous embryonic
cell types in the aggregates was tested by aggregating normal pluripotent cells
taken from 3.5 day embryos with 2.5 day embryos. Early embryo loss was very
high, and histological studies showed that the majority of these embryos died by
6.5 days development. Some embryos escaped this early death such that some
healthy chimeras were recovered, in contrast to recovery of abnormal chimeric
embryos following Pl0-morula aggregations, and non-chimeric embryos following
P19-morula aggregations.
This somewhat surprising adverse effect on development following
aggregation of normal cell types suggests that there are developmental
difficulties associated with the mixing of asynchronous cell types in
aggregates. However, the greater magnitude of the adverse effects when the
aggregates contained tumor derived cells suggests that EC cells should not be
considered the complete equivalent of the pluripotent cells of the early
embryo.